What is the difference between
No Action, e.g. the Monk's
Centered Flurry of Blows [ddi], and the more common
There are conditions that prevent characters from taking actions altogether (stunned, unconscious, dying, etc). This prevents free actions, but a "No Action" power could still be used. Also note that most powers that enhance your initiative are No Action because you are using them before combat technically begins.
My guess at the reason this particular power is a No Action power is that you can normally only make one attack as a free action per turn. "No Action" powers don't count toward this limit.
Centered Flurry of Blows could be argued to be an attack power, and thus use up your 1 free action attack per turn. Giving it a "No Action" ensures that it doesn't.